1. Field of the Invention
The present invention relates to image forming apparatuses, such as copy machines, facsimile machines, and printers, and to image forming methods.
2. Description of the Related Art
In an image forming apparatus based on an electrophotographic technology, coloring particles of toner are used to produce a toner image on a recording material surface in an image formation step. This is followed by a fusing step in which the formed toner image is fused onto the recording material, wherein the toner is melted by heat and then solidifies upon cooling.
A conventional fusing device for fusing a toner image on the recording material surface includes a fusing roller and a pressure roller. The fusing roller is a fusing rotating body that has a heating unit, such as a halogen heater, disposed inside the rotating body. The pressure roller is a pressure rotating body that is pressed against the fusing roller by a coil spring or the like, forming a nipping portion between the pressure roller and the fusing roller. In such a fusing device, a recording material carrying a toner image on the surface thereof is passed through the nipping portion between the fusing roller and the pressure roller, and heat and pressure are applied in order to fuse the toner image on the recording material.
In this type of a fusing device, in order to fuse the toner image onto the recording material, such as a transfer sheet, without damaging the image quality, it is necessary to maintain a target temperature of the fusing roller, by which an amount of heat is supplied to the toner image at the nipping portion. If the temperature is either too high or too low, an image defect called “offset” or “fusing defect” may be caused. Thus, it is important to maintain an appropriate fusing temperature of the fusing roller.
Thus, the temperature on the surface of the fusing roller is detected by a temperature sensor as a fusing temperature detecting unit, and the heating unit is controlled based on a detected result so that an appropriate fusing temperature can be maintained.
However, immediately after the turning on of power, the temperature of the pressure roller may be low even when the fusing roller surface has reached a predetermined temperature through a warm-up operation. This is due to the fact that, because the fusing roller is heated to the predetermined temperature without rotating the pair of the fusing roller and the pressure roller, only a portion of the surface of the pressure roller that is in contact with the fusing roller is heated, leaving the other surface portions of the pressure roller unheated. As a result, when the pair of the fusing roller and the pressure roller are rotated in response to an image formation request immediately after a warm-up operation, the portions of the pressure roller that have a lower surface temperature come into contact with the surface of the fusing roller, whereby the heat of the fusing roller is taken away and the surface temperature of the fusing roller decreases.
Eventually, as the pressure roller surface is heated, the surface temperature of the fusing roller rises to the predetermined temperature. However, the recording material may have already passed the nipping area when the surface temperature of the fusing roller has recovered, thereby resulting in a fusing defect.
Japanese Laid-Open Patent Application No. 8-262896 discloses that a pressure roller temperature is estimated from the difference between an actual surface temperature of the fusing roller when it is heated by a warm-up operation, and a target temperature of the fusing roller. When the estimated pressure roller temperature is too low, the time between an image formation request and the arrival of the recording material with a toner image formed thereon at the fusing/nipping portion is extended.
By thus increasing the time between the instruction for image formation and the arrival of the recording material at the fusing/nipping portion, the surface temperature of the fusing roller that has dropped due to the transfer of heat to the pressure roller can recover back to the target temperature when the recording material passes through the fusing/nipping portion.
In this way, the development of fusing defect can be prevented. In this conventional technology, when it is estimated that the temperature of the pressure roller is sufficiently high, no extension of time is made between the reception of an instruction for image formation and the arrival of the recording material at the fusing/nipping portion, so that the first print time can be shortened.
However, in the above-discussed conventional technology, the temperature of the pressure roller after the warm-up operation may not be correctly estimated due to various factors, such as the eccentricity in the fusing roller or the pressure roller. For example, if there is eccentricity in the fusing roller, for example, the nipping width when the roller is stopped varies. When the nipping width in the absence of rotation is narrower, less heat taken is away by the pressure roller. Thus, the actual surface temperature of the fusing roller decreases less when the fusing roller is heated through the warm-up operation to a target temperature. As a result, it is wrongly estimated that the temperature of the pressure roller is sufficiently high when in fact the temperature of the pressure roller is low.
If this happens, the surface temperature of the fusing roller greatly decreases once the rollers are rotated, and yet the time before the arrival of the recording material at the fusing/nipping portion is not extended even though such extension of time is required for the surface temperature of the fusing roller to reach the target temperature.
Consequently, the recording material arrives at the fusing/nipping portion before the surface temperature of the fusing roller has recovered sufficiently, thereby resulting in a fusing defect. Particularly, when the fusing roller has a high thermal responsiveness, the actual temperature of the fusing roller surface may greatly vary due to factors such as the eccentricity in the fusing roller or the pressure roller, thereby increasing the probability of the development of a fusing defect.